The present invention relates to a planarized-packing device and structure for pyroelectric IR sensing elements and, in particular, to such a device and structure having a greater viewing angle for sensing.
Recently, heat-sensing security monitoring systems have become increasingly popular. The popularity of such systems is substantially the result of the development of pyroelectric IR heat-sensing elements. Pyroelectric IR heat-sensing elements generate a corresponding electrical signal in response to the IR heat wave radiated from an approaching person or heat source. The electrical signal is then amplified for use by the preventive alarm or triggering circuit of a security system. Pyroelectric IR heat-sensing elements have the following advantages:
1. The pyroelectric effect is a differential function of time so that it provides automatic background (static background heat source) suppression, thus effecting dynamic sensing and simplifying signal processing;
2. Pyroelectricity is generated spontaneously without the application of bias voltage, thus avoiding noise caused by bias voltage; and
3. The elements are made using the same simple techniques as those used for making typical ceramic capacitors. They can be made by such reduced-cost techniques as coating with electrodes and vapor depositing with black body film.
The above advantages make it feasible to provide inexpensive non-contact radiation heat-sensing elements capable of operating at room temperatures. In recent years, such heat-sensing elements have been widely used in burglar and fire prevention systems and in sensors for power-saving light controllers. As a result, more and more studies in pyroelectric IR heat-sensing elements are presently being undertaken.
In general, heat sensing elements must be made as thin as possible so as to reduce heat capacity. At the same time, they have to be made in a suspended structure so as to reduce heat loss, i.e., increase thermal impedance. If made this way, when the elements absorb heat energy, they can produce a higher temperature rise, i.e., have a higher thermal response, and thereby provide a greater pyroelectric response. Unfortunately, requiring the elements to have a thin and suspended structure causes the adhering technique needed for their assembly to become complicated. In short, much improvement is needed in making the radiation heat-sensing elements having these complicated structure and device requirements.
In FIG. 1, there is shown the equivalent circuit of a conventional two-piece pyroelectric sensing element (in phantom lined box) wherein pyroelectric sensing elements A1 and A2 may be made of thin sheets of a pyroelectric material such as LiTaO.sup.3, PZT, PVF2 or TGS. The pyroelectric sensing elements A1 and A2 each are plated with electrodes on both sides thereof and each have their upper surface coated with a black body film for increasing the heat absorbing effect. The two sheet sensing elements are opposite in direction of polarity as shown by the arrows, with one facing up and the other facing down. The static charge or voltage resulting from heat induction in elements A1 and A2 is transferred to a field effect transistor (JFET) Q of high input impedance which serves as an impedance buffer for an external circuit. The field effect transistor Q outputs a signal from a terminal L2. A load resistor RL of high resistivity is provided in parallel between the output ends of the sheet sensing elements A1 and A2 to balance the leakage of the pyroelectric induced charge and the transmission of low frequency noises, and to select the bandwidth for the circuit. Two heat sensing pieces are provided in order to eliminate the microphonic noise caused by vibration and to reject the common mode signal resulting from changes in background heat radiation. The assembly of the above elements must be placed in a sealed metal container to prevent reduction of signal-to-noise ratio due to interference from electromagnetic noises.
FIG. 5 shows two-piece heat sensing element `S` used in a heat inducing system having a multiple IR focusing leans array L. The lens array L in front of the element S divides the field of view of the system into a plurality of small fields of view (shown in slanted lines) such that any heat wave generated by a moving heat-emitted body, e.g., the heat wave from a person, may be imaged alternatively on and absorbed by the two pyroelectric sensing elements. This causes alternative temperature rise and fall in the two elements, thus generating a differential output signal in the frequency of about 1 to 10 Hz.
FIGS. 2 and 3 show, respectively, the structures of two types of presently commercially available pyroelectric sensing elements. In the conventional structure shown in FIG. 2, a pair of strip-like insulating pads 102, 103 are adhered to a metal base 101, and then a pair of pyroelectric sheet elements A1 and A2 (as shown in FIG. 1) are bridged thereon at a position where the optical system is focused. In addition, other elements such as the field effect transistor JFET and the load resistor RL as shown in FIG. 1 have to be provided on an insulating pad 104 affixed to the base so as to prevent short-circuit to the metal base 101. After this complicated structure is formed, ultrasonic wire welding such as that practiced in conventional semiconductor device packaging techniques is carried out to make the necessary circuit connections. Then, the structure is sealed by having a cover 106 welded thereto. A multi-layer silicon long wavelength infrared (LWIR) filter 105 is adhered to the cover for transmitting heat waves having 6-14 micron wavelengths while suppressing background light rays of other wavelengths.
In the structure of another conventional pyroelectric sensing element as shown in FIG. 3, a plurality of arcuated welding wire rings 108 are further provided on a ceramic insulating pad 107, and then pyroelectric sheet elements (diagrammatically shown by reference number A) are adhered to the upper ends of respective welding wire rings 108 to obtain a suspended heat isolation structure.
In both of the conventional pyroelectric element structures shown above, a stacking approach is utilized for suspension and heat insulation purposes. This approach causes the packaging of the elements and the structure of the products to become relatively complicated and unsuitable for automated assembling operation, thus resulting in high labor cost. Moreover, the metal base is rather expensive, accounting for a relatively high percentage (about 25%) of the cost of the materials, and insulating pads must be provided to prevent conduction to the base, both substantially increasing the manufacturing and processing costs.
To cope with such adverse factors, a special planarized-packing structure as shown in FIG. 4A is proposed (see ROC Utility Model Patent No. 67164) in which a double-sided circuit board 201 is formed with a conductive copper foil 204 having a special circuit layout and connection such that the elements as shown in FIG. 1 can be disposed by using the surface mounting technique (SMT). A plurality of through holes 202 are set in the circuit board 201 for pins 203 to pass through. An automatic riveting technique is used such that the pyroelectric elements are placed accurately at desired positions during an assembling operation. The structure of said ROC Utility Model No. 67164 is characterized by an undercut well 205 which is set on the region of the circuit board where the sheets of pyroelectric material A1 and A2 are provided so that the sheets of pyroelectric material A1 and A2 bridge over it. The undercut well 205 is also spread with the copper foils 204. A side cross section view of the structure is shown in FIG. 4B. With the pyroelectric elements A1 and A2 bridged over the region of the well 205, a suspended framework is formed, thus providing excellent heat isolating effects. Furthermore, since such a suspended structure allows the thin pyroelectric sheet elements A1 and A2 as well as other added elements to be all arranged on the same plane of the circuit board, an automated assembling operation can be performed using the surface mounting technique and equipment. Moreover, because of the electrical insulation of the circuit board itself, there is no need for additional insulation pads. Also, since the circuit board can be laid out for various applications, the required number of ultrasonic wire welding is reduced. In addition, the double-sided circuit board can effectively prevent interference from electromagnetic noise.
After assembly and test, the structure in FIGS. 4A and 4B can be fitted into a cover casing. As shown in FIG. 4C, the top of the cover 206 has an IR window 207 which receives multi-layer thermal IR filtering plate 208 for permitting the radiative thermal energy to pass through. The cover 206 also has flanges on the inner edge of the cover (not shown) so that when the circuit board 201 abuts exactly against said flanges to fix a relative distance between the pyroelectric elements A1 and A2 and the IR window 207 for precise optical sensing. Finally, over the side of the circuit board 201 opposite to that fitted into the cover 206, an epoxy resin is injected and thermally cured for airtight sealing. The cover 206 can be made of metal formed by a pressing technique and then plated with metal to obtain the electrical, noise isolation and water resistance benefits.
Moreover, no pins are needed for the through holes 202 of the circuit board 201. After having been assembled and tested, the structure shown in FIGS. 4A and 4B can be welded to the pin ends (the elements 210 shown in FIGS. 6A and 6B) of a conventional TO metal base so as to float on the base surface. Then a metal cover having a filter window is welded using standard TO metal packaging equipment to obtain the finished product.
As can be appreciated from the foregoing, the prior device and structure (ROC Utility Model Patent No. 67164) has the following advantages:
1. All the elements are adhered on the plane of one circuit board. Therefore, automated assembly using currently available surface bonding equipment can be employed so as to achieve the benefit of quantity production at lowest cost.
2. The pre-pressing undercut well 205 enables the pyroelectric elements placed thereon to be implemented in a suspended structure without requiring insulating pads, thus reducing the costs of processing and materials.
3. The circuit board is used as the base, eliminating the need for an expensive metal base.
4. Insulating pads are no longer required between the sensing elements and the metal base, thus reducing material and processing costs.
5. Layout connection on the circuit board enables the required wire welding to be reduced.
However, all the conventional structures, including the above R.O.C. Utility Model Patent No. 67164, have the disadvantage that the widest viewing angle available to their heating sensing elements is limited no more than 110 degrees. In order, to increase the viewing angle, the cost of the materials and the volume of the structure have to be increased. The reasons are described by reference to FIGS. 6A and 6B as follows:
In FIG. 6A there is shown the schematic cross view of the packaging structure of R.O.C. Utility Model Patent No. 67164. From the light rays rm1 and rm2, it can be seen that the sensing devices of such a geometry have a maximum viewing angle .THETA.m, generally about 110 degrees for presently available products. In application, the viewing angle of such a structure can hardly cover the angle of a half circle, i.e., 180 degrees, such as that bounded by a wall (as the diagram shown in FIG. 5). Two approaches can be used to increase the view angle:
The first approach is to enlarge the dimensions of the viewing window 207 for the filtering plate 208, shown by reference letter W in the figure. This, however, will increase both the material cost and the packaging volume because the filtering plate 208 transmissive to thermal IR heat waves of 8-14 micron wavelength are made of expensive silicon ships which must be coated with tens of layers of germanium and zinc sulfide films. The fabrication of this plate is therefore time-consuming and quite expensive. Moreover, with the dimensions being increased, the metal base has to be enlarged, further increasing the volume and the material cost.
The second approach is to reduce the distance H, as shown in FIG. 6A, between the sensing elements A1 and A2 and the filtering plate 208. This approach avoids the disadvantages of the first approach. However, a welding wire 209 must be applied on the upper surface of the elements A1 and A2 to connect them with the external circuits; accordingly, a gap must exist for the welding wire 209. Therefore, the volume of such a structure needs to be enlarged and the viewing angle can not be improved properly. In short, increasing the viewing angle has unfavorable effects on the cost and volume of the elements.